7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Investigating the temperature dependence of helium bubble dynamics in plasma
exposed tungsten via in-situ TEM annealing
Soon Han Bryan Teo1, Matt Thompson1, Maryna Bilokur1, Dhriti Bhattacharyya2,3, Cormac Corr1
1
Department of Materials Physics, Research School of Physics, Australian National University.
2
Australian Nuclear Science and Technology Organisation.
3
School of Materials Science and Engineering, University of New South Wales.
E-mail (speaker): soon.teo@anu.edu.au
The International Thermonuclear Experimental Reactor’s(ITER) decision to operate with a full tungsten (W)divertor has spurred further interest in the material’s
prospects[1, 2]. W is the favoured plasma-facing material
in fusion reactors such as ITER due to its high melting
point and low tritium retention. Given the harshconditions in a reactor plasma-facing component’s
lifetime remains a crucial aspect of study to minimize
interruption to tokamak operations[3, 4].
In-situ TEM annealing and imaging were performed onpolycrystalline bulk W samples pre-exposed at either 573
K or 1013 K to a pure helium (He) plasma to a fluence of1025 m-2 to investigate the thermal evolution of He bubbles.
It was found that annealing temperatures had little effecton the bubble dynamics for the lower temperature
exposure of 573 K. Elongated bubble structures remainedin the sample throughout the annealing process. There isminimal change in the average bubble size and numberdensity. However, annealing had a significant effect on thehigher temperature exposure of 1013 K. Bubbles werefound to be trapped in large clusters, and increasing theannealing temperature caused a significant increase in the
average bubble size accompanied by a decrease in bubble
number density. The experimental observations aresupported by theoretical calculations developed byHammond et al. that show a simultaneous increase in the
average bubble size and an overall decrease in He atoms
retained within bubbles as the annealing temperature
increased for the higher exposure temperature sample.
These trends in the bubble dynamics with annealing
temperature suggest that Ostwald ripening dominates
annealing bubble growth at annealing temperatures up to1000 K, whereby bubble growth is determined by thedifferent pressures of different bubble sizes rather thanthrough bubble migration and coalescence.
References:[1]: Mitteau, R., et al., The design of the ITER first wall
panels. Fusion Engineering and Design, 2013. 88(6-8):
p.568-570[2]: Pitts, R.A., et al., A full tungsten divertor for ITER:
Physics issues and design status: Journal of Nuclear
Materials, 2013. 438: p. S48-S56.
[3]: Roth, J., et al., Recent analysis of key plasma wallinteractions issues for ITER. Journal of nuclear
materials, 2009 390: p.1-9[4]: Pitts, R.A., et al., Physics basis for the first ITER

tungsten divertor. Nuclear Materials and Energy, 2019
20: 100696

Figure 1:1 µm under-focus TEM micrographs of in-situ annealed W
exposed to He plasma at 1013 K. The annealing temperatures was(a) room temperatures (f) room temperature after the sample washeated and held at 998 K for 20 minutes.

Figure 2: Histogram of the measured cross-sectional area ofindividual bubbles pre-annealing and post-annealing at 998K after
20 minutes in the high-temperature exposure sample at 1 µm
under-focus, fitted to a log-normal distribution.